Since the only data NASA can send is the direction the camera is facing, Mark comes up with a system where they can point at cards with alphabet characters on them to type messages. But using the letters 'a-z' would be impractical. To quote the book (from this answerthis answer, over on scifi.se):

Since the only data NASA can send is the direction the camera is facing, Mark comes up with a system where they can point at cards with alphabet characters on them to type messages. But using the letters 'a-z' would be impractical. To quote the book (from this answer, over on scifi.se):

Since the only data NASA can send is the direction the camera is facing, Mark comes up with a system where they can point at cards with alphabet characters on them to type messages. But using the letters 'a-z' would be impractical. To quote the book (from this answer, over on scifi.se):